Atomic Layer Epitaxy (ALE) and Atomic Layer Deposition (ALD) have demonstrated the ability to provide layer-by-layer control in the deposition of atoms and/or molecules for many material systems using a variety of techniques. In each of these cases, the deposition process is cyclical, where a repeatable portion (less than, equal to, or somewhat greater than) of a monolayer is deposited with each full cycle. There are both gas phase and liquid phase ALE or ALD techniques for a wide range of materials. ALE or ALD processes have been developed for elemental semiconductors, compound semiconductors, metals, metal oxides, and insulators. There are ALE processes that involve the deposition of a self-limiting monolayer per cycle. For example, with gas phase gallium-arsenide (GaAs) ALE, a single monolayer of gallium (Ga) is deposited on a GaAs surface from trimethylgallium (TMG), where methyl groups passivate the gallium surface and limit the deposition to a monolayer. The TMG is pumped out and arsine is used to deposit a self-limiting layer of arsenide (As). The cycle is repeated to produce several layers of GaAs. It is also possible to substitute trimethylaluminium (TMA) instead of TMG. In this way, layered epitaxial structures, i.e., structures comprised of multiple layers, each layer in one monolayer, thus achieving atomic precision.
Various attempts have also been made for providing lateral patterning at the atomic level. One example involved using a scanning tunneling microscope (STM) to push xenon atoms across a nickel surface. Another example involved an approach for creating patterns in hydrogen atoms adsorbed on a silicon surface by getting them to desorb from the surface with electrical current from an STM that pumps energy into the silicon-hydrogen bond. The ability to selectively depassivate surfaces by removing an adsorbed monolayer of hydrogen is significant because there are ALE approaches which employ an adsorbed hydrogen layer as the self-limiting process.